The present invention is directed to thermoplastic containers and, more particularly, to a thermoplastic container specifically adapted for packaging meat and other foodstuffs.
Cellular or foam trays currently are widely used for packaging fresh meat and other foodstuffs. Typically, meat is placed in the tray and overwrapped with a highly elastic, amorphous polymeric film. Even under refrigeration, this type of packaging provides only a limited shelf life for the foodstuffs, typically no more than about seven days. As unsold products approach the end of their shelf life, grocers often need to discount the sale price, and in many cases discard products whose shelf life has expired. The stretched lid stock also exerts stresses on the walls of the cellular or foam tray, which can result in deformation of the tray and/or delamination of the lid stock.
Another problem with currently available trays is consumer misuse. The cellular or foam trays generally are not designed for cooking and generally are not high temperature resistant. One common instance of consumer misuse involves defrosting or thawing frozen meat, while in the tray, in a microwave oven. Because the tray is not designed or intended for cooking, potentially toxic components of the tray can be liberated into the food when the tray containing the foodstuff is exposed to heat or microwave radiation.
It would be desirable to develop a container for packaging meat and other foodstuffs that provides increased shelf life. It also would be desirable to develop a tray for packaging meat that is microwave-safe. It would be particularly desirable to develop such a container that can be manufactured in a cost effective process.
The present invention is directed to a multi-layered thermoplastic container having a first polymeric layer, a second intermediate layer, and a third polymeric layer. The container is particularly useful for packaging foodstuffs (e.g., fresh meat, fish, or vegetables, prepared or semi-prepared foods, and the like) using gas flushed sealing or modified atmosphere packaging (MAP) with highly elastic films. The polyalkylene terephthalate or naphthalate first layer provides stiffness and dimensional stability to the container, thereby avoiding deformation due to stresses, for example from the stretched lid stock. The first layer also provides excellent gas barrier properties to the container. The third polymeric layer provides markedly improved adhesion with conventional polyethylene-based lid stock. The container, as a whole, exhibits significantly improved gas barrier properties and shelf life for packaged foodstuffs.
The first polymeric layer comprises an alkylene terephthalate or naphthalate polyester, such as PET, and optionally contains recycled or reprocessed polymers.
The second intermediate or tie layer functions primarily as an adhesive and as a compatibilizer/emulsifier/surfactant (CES), and comprises a grafted or backbone co-polymer or ter-polymer of ethylene and a glycidyl acrylate, maleic anhydride, or mixture thereof, and optionally an acrylate co-monomer selected from the group consisting of methacrylate, ethylacrylate, propylacrylate, butylacrylate, ethylhexylacrylate, and mixtures thereof. The second layer optionally comprises a blend of the CES and a polar copolymer of ethylene and a C1-C12 acrylate, such as methacrylate, ethylacrylate, propylacrylate, butylacrylate, ethylhexylacrylate, or a mixture thereof. Alternatively, the second layer comprises a suitable adhesive material, for example an anhydride modified polyolefin such as Bynel (duPont); and a minor amount of the CES, usually from about 1 to about 5 wt %, is blended with the polyester first layer.
The third polymeric layer comprises high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), or a blend thereof. Any or all of the polyethylene materials in the third layer can be metallocene-based polyethylene. Metallocene-based polyethylene materials generally have lower melting points and are more amorphous, which improves heat-sealing properties. The third layer usually forms the inner surface of the container, and preferably exhibits good sealing properties with lidding materials.
The container of the present invention can contain additional layers, including a multiplicity of any or all of the aforementioned layers, in any order, as well as one or more additional layers for a wide variety of purposes, including but not limited to aesthetics, color, gloss, physical or chemical compatibility with other layers, and barrier properties. There is no upper limit contemplated on the number of layers that can be present.
The multi-layered container of the present invention provides dramatically improved barrier properties, yielding increased shelf life for food products such as fresh meat, fish, and vegetables. The shelf life of foodstuffs packaged in the container can be as much as 30 days or more, as compared to about 7 days with current trays. The container of the present invention is microwave-safe, i.e., will not contaminate the food upon exposure to heat or microwave radiation. In an alternative embodiment of the invention, the container, though microwave-safe, visibly distorts upon exposure to heat, for example the heat produced by food inside the container being exposed to microwave radiation, so as to provide a dramatic indicator of consumer misuse in applications where the container is not intended for cooking.
Alternatively, the materials of the container can be thermally treated to permit the container to have suitable high temperature resistance to permit use of the container in various cooking applications, such as thawing frozen foods in a microwave oven.